How do I fix this IsInGroup is a nill value?

You are doing Clicker.Name, which is a string of course. It would obviously error to do a_string:IsInGroup(). You would just need to change that to Clicked, which is the "Instance" that clicked. So the right thing is Instance:IsInGroup And that's also for the :GetRankInGroup

Another thing, in the :GetRankInGroup bit, your putting the rank as the paramater, while the paramater for that function should be the groupId so fix that.

local rank = 1

local groupID = 4857556

  

clicked = false

waited = true

  

function onClicked(Clicker)

print(Clicker.Name)

if (Clicker:IsInGroup(groupID) and Clicker:GetRankInGroup(groupID) >= rank) then

  

if clicked == true and waited == true

then

script.Parent.Transparency = 0

script.Parent.CanCollide = true

clicked = false

print("Clicked Close")

wait(0.1)

waited = false

end

  

  

  

if clicked == false and waited == true

then

script.Parent.Transparency = 0.5

script.Parent.CanCollide = false

clicked = true

end

wait(0.1)

waited = true

 
end
end
script.Parent.ClickDetector.MouseClick:connect(onClicked)